Suppose sender of anthrax letters took Saudia flight from Saudi Arabia to New York. He enters the US with a forged Yemini passport. The passport never been used before and there is no connection to the assumed name from any other terrorist operation. He also has credit cards, driver's license, etc.
Then he rents a car from Hertz and drives to Brooklyn. There he purchases pre-stamped envelopes at a post office in a neighborhood with a primarilly Middle Eastern population. He then writes the letters in question on paper he obtained at any stationary store. He then opens his bottle of colone, toothpaste tube, can of shaving creme or other container containing the anthrax and places the spores within the letters. He then places the letters inside a "baggie." The entire transfer could take place in the sink of a cheap motel room. He then washes the sink very well with commercially available bleach.
The motel, paper and envelopes were purchased with cash.
He then drives to Trenton, NJ. His trip would take about 3 hours at most. He eats at fast food drive-throughs. The trip would be easy, although he avoids the NJ Turnpike so his license number is never recorded by a toll booth camera. Upon arriving in Trenton, he then deposits his anthrax laden letters in a street corner mailbox. He replaces the baggie in the car. He then returns to New York, purchases five gallons of gasoline and completely burns the car, inside and out. Also consumed by the fire are the baggie, original anthrax container, his gloves, disposable biohazard suit, etc. He is never worried about contracting anthrax because he has been taking a heavy propholactic dose of Cipro for more than a week.
He then takes public transportation back to JFK and boards the next flight to Saudi Arabia. His total time in the US; less than 28 hours. Upon arriving in Saudi Arabia, he clears customs then burns the phony passport and all other items containing that name.
By that time, the NYC Department of Sanitation has towed the burned out car, like they do 80 burned out cars every day, to its junkyard and begins the 10 day process of notifying Hertz. The baggie, gasoline jug and bioharard suit and remaining spores were consumed in the fire and are never noticed.
He then returns to Afganistan, where he reports his success, which by now is beginning to be covered by CNN. Senior Bin Laden representatives shoot him and burn his body. He is glad to die for Allah.
He never existed. His case is not solvable.
What case?
The next day, a man boards a flight in Manila.....
Here's where your hypothesis falls apart. All flights in/out of NYC were grounded for days. I think this was accomplished by the hijackers themselves, or one of their compatriots that they left behind. Remember all of the Middle Easterners who suddenly fled the country in the week after the attack? And there would be no record of anyone who managed to cross either the Canadian or Mexican border on foot.